Fix a crash that decoder job might be deleted when data arrives

During tab switching, a release() might be called first before OnDemuxerDataAvailable().
We need to check the decoder job before passing the data to them

BUG=299887

Review URL: https://codereview.chromium.org/25020003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226688 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226688